The field of the invention is arc lamps such as metal-halide, sodium, mercury or rare gas lamps.
High-pressure arc lamps (including metal-halide, sodium, mercury, and rare gas lamps and often referred to as high intensity discharge lamps) currently produce, on a continuous basis, several billion watts of visible light worldwide. A major factor limiting the life of these lamps is the process of starting and re-igniting the lamp. The startup or re-ignition of an arc lamp requires the production of charge carriers and, in conventional lamps, this is normally done by imposing a large voltage across the lamp. This voltage creates an electric field between the discharge electrodes that ionizes a fraction of the gas atoms or molecules in this region. The positive ions (anions) and electrons produced by ionization are drawn, under the influence of the electric field, to the cathode and anode, respectively, giving rise to a current. Near the cold cathode, the anions normally are responsible for carrying most of the current. As they approach the cathode, the anions are accelerated in a strong electric field and, upon impact with the cathode, cause the release of secondary electrons from the cathode. In the early stages of the start up of an arc lamp this is the dominant source of electron production near the cathode. Unfortunately, when an ion collides with the cathode, sputtering of the cathode also occurs. Some of the sputtered cathode material (generally a metal or alloy such as thoriated tungsten) coats the inside of the envelope of the lamp, thereby reducing the fraction of the light produced by the lamp that is able to escape. This process is called wall darkening. Each time the lamp is ignited or re-ignited, the operating life of the lamp is reduced further by this process.
The invention assists in the cold startup or re-ignition of a high-pressure arc lamp. One or more microdischarge devices produces and injects electrons and ions into the region between the cathode and anode. The microdischarge device(s) do not serve as the cathode itself but augment electron and ion production in the early stages of lamp ignition or re-ignition. Once the glow (and arc) is firmly established between the anode and cathode, the microdischarge device(s) can be extinguished and the lamp will function in a conventional manner.